1. Field of the Invention
The present invention relates to an engaging device, such as a multiple disc clutch or multiple disc brake, and in particular to such an engaging device incorporated in an automatic transmission.
2. Description of Prior Art
As one type of the engaging device, there has been known a braking device of an automatic transmission as disclosed in Laid-open Publication No. 2-48650 of Japanese Utility Model Application.
FIG. 6 is a vertical cross sectional view showing the braking device of an automatic transmission, and FIG. 7 is a cross sectional view taken along VII--VII line in FIG. 6.
This braking device is incorporated in an automatic transmission in which a torque converter is linked to a change-gear mechanism using a planetary gear device. The change-gear mechanism includes a plurality of planetary gear sets, clutches, and brakes. These clutches and brakes are engaged in different combinations, so that the plurality of planetary gear sets generates a shifting output to establish a desired change gear ratio.
As shown in FIG. 6, the known braking device includes a spline 64 formed in the inner surface of a circumferential wall of a transmission case 60 and held in mesh with a plurality of friction plates 63, and a plurality of friction plates 62 alternately overlapping with the friction plates 63 and held in mesh with a spline formed at the outer periphery of a ring gear 61. A seat rest for a spring seat is formed at the rear end face of the spline 64. Spring seat 65 engages with and is fixed to this seat rest.
A cylinder 68 is fixed via a seal member to the inner surface of the circumferential wall of the transmission case 60, and an annular hydraulic chamber 67 is formed between the inner surface of the circumferential wall and the cylinder 68. A ring-shaped brake piston 69 for pressing the friction plates 62, 63 is provided inside the annular hydraulic chamber 67 such that the piston 69 is freely slidable in the axial direction. Also, a plurality of return springs 70 for biasing or forcing the brake piston 69 in a direction away from the friction plates 62, 63 is disposed between the spring seat 65 and the brake piston 69, such that the return springs 70 are spaced apart from each other in the circumferential direction of the transmission.
As shown in FIG. 7, the plurality of return springs 70 are disposed in three groups 70A, 70B, and 70C that are spaced apart from each other at substantially equal intervals in the circumferential direction. Each group includes seven return springs 70 that are spaced from each other at substantially equal intervals in the circumferential direction.
In the above-described prior art, a pressing face of the brake piston 69 that presses the friction plates 62, 63 is divided into three parts 69A, 69B, 69C that are spaced apart from each other at substantially equal intervals in the circumferential direction, so as not to interfere with the return springs 70 of each of the three groups 70A, 70B, and 70C. In this arrangement, the friction plates 62, 63 are locally pressed at three locations by the thus formed pressing parts 69A, 69B, 69C of the brake piston 69, respectively. As a result, the friction plates 62 and 63, which are made of thin metal plates, are deformed, and do not contact uniformly with each other over the entire surfaces, namely, only local portions of the friction plates 62, 63 pressed by the brake piston 69 contact with each other. Consequently, the friction plates 62 and 63 may wear severely, with a risk of seize-up and other problems, and the durability of the brake disc is undesirably lowered.